Air Force

TPS students analyze C-12 Huron rudder forces

The tufting visible on the tail of the C-12 Huron played a critical role in the SPINAL TAP test management project. It allowed the team to investigate the force-lightening anomaly using flow visibility. Testing was done during a series of maneuvers performed over the course of six sorties.

Karl Major, a civilian test pilot at the United States Air Force Test Pilot School, has been studying and flying the C-12 Huron for years. Over the years, he noticed an interesting phenomenon with the aircraft while experiencing sideslip.

Under certain conditions, the more the pilot would push on the rudder pedal to generate sideslip, the less the force the pilot would have to apply to get the rudder pedal to deflect. This was not intuitive and not a very desirable characteristic. Hoping to learn more information about the force-lightening anomaly, Major tasked USAF TPS students to investigate the situation further.

“As students go through TPS, they learn the theory of how and why an aircraft behaves in a certain way. It is important to their education to understand how different aspects might affect the flight characteristics,” said Major. “In this case, TPS students will discover the C-12’s rudder force anomaly on recurring curriculum rides.”

The project is the culmination of the students’ intensive year-long Masters Program at the USAF TPS.

According to David Vanhoy, USAF TPS technical director, completing the test management project presents unique challenges for students who must balance the successful completion of the project with meeting additional curriculum-based requirements.

Air Force photograph by Chris Neil

The test management project SPINAL TAP, short for Science Project in Aircraft Laboratory, Tufted Aircraft Panels, began last December hoping to investigate the phenomenon with flow visibility on a tufted vertical tail. Two sorties were videotaped from a C-12 chase aircraft, allowing the team to review data and visually identify how the tail of the aircraft responded during the various airspeeds and aircraft configurations.

“It’s truly amazing what our students are able to accomplish,” said Vanhoy. “And the SPINAL TAP folks are no exception.”

The test management project SPINAL TAP, short for Science Project in Aircraft Laboratory, Tufted Aircraft Panels, began last December as the team prepared to look into the phenomenon by using a flow visualization technique on the C-12’s tufted vertical tail.

Tufting refers to the strings of yarn attached to the surface of the aircraft’s tail, which allowed the students to visually monitor their activity during flight, particularly when performing maneuvers that induced sideslip to the aircraft. This technique shows locations where the airflow is attached to the surface of the tail, where it has separated from the surface, and the general direction the air is flowing along the surface, all phenomena that are otherwise invisible to the eye.

“Looking at one subject in greater detail can be extremely valuable,” said Baughman, USAF TPS, SPINAL TAP project manager. “The safety of the C-12 rudder was never in doubt, but it is certainly an interesting characteristic and we were excited to have the opportunity to look further into it.”

Steady heading sideslips, rudder doublets, asymmetrical thrust, and the “Webster” flight test maneuver, created by the students, were used to induce sideslip while testing various airspeeds and aircraft configurations.

According to Baughman, the Webster flight test maneuver was named after the group’s technical advisor, Fred Webster who works with the 773rd Test Squadron at Edwards. The maneuver was designed to gather data that Webster built software to analyze.

Of the four maneuvers performed during flight, the steady heading sideslips turned out to be the most effective for gathering data.

“Normally as a pilot, I would continue pushing on the rudder pedal to get out to where I need to be,” said Baughman. “But, if I were to instantaneously push on the pedal to get a fixed amount of sideslip, then that’s where you start seeing the pedal force lightening in the C-12. The steady heading sideslip maneuver is the best way to see this happen since we’re taking snapshots at different angles of sideslip.”

Early on in the project, it became clear that there were physical challenges associated with the SPINAL TAP test management project. Performing steady heading sideslips required pilots to apply approximately 150 pounds of force on the rudder pedals for extensive periods of time throughout each of the six, two-hour long sorties.

“We quickly realized that flying these maneuvers turned out to be quite a workout,” said Baughman. “We decided to take two pilots on each sortie so that we could alternate and get a much needed break.”

Two of the six sorties were videotaped from a C-12 chase aircraft, allowing the team to review data and visually identify how the tail of the aircraft responded during the various airspeeds and aircraft configurations.

The team’s preliminary findings indicated that the anomaly is more subtle than the force lightening that Major and the students had originally thought.

“When we started the testing, force lightening showed up in the data right away,” said Underwood. “However, as we continued testing, we started to see different results.”

The data also seems to indicate that aircraft configuration plays a much larger role in force change than airspeed.

“When we introduced flaps into the aircraft configuration, we saw more sideslip and more flow separation,” said Baughman.

With the flying portion of the project completed, the team will now focus on the data and aerial footage to identify the precise moments when the tufts on the aircraft tail indicate a change in flow characteristics.

The SPINAL TAP test management project team will present their findings to the USAF TPS in May before their graduation in early June.

Their project was certainly an interesting one,” said Vanhoy. “It was truly a blend of modern and historical approaches to flight test. I can’t wait to see how their results turn out.”

According to Major, the results of the experiment will benefit future Test Pilot School students and could possibly even spark interest into further investigation of the force change anomaly of the C-12 Huron.

“The results of the tests will definitely be folded back into the curriculum for our students. In addition, the results could lead to further inquiries if new questions are discovered during the team’s analysis,” said Major. “The broader and deeper the knowledge base of our students, the better they will perform as airborne evaluators of new or highly modified aircraft.”

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